Add purge of msg queue to send_all method.

- Add purge of message queue after sending all messages to avoid empty
  intermediate directories filling up top directory.
Add OSError catch block to message queue purge.
Clarify language in log messages.

- Remove 'queue' from 'message queue was locked' as it is the message not
  the queue being locked.
- Change 'queue' to 'directory' during purge to make it more
  understandable to the user.
Merge pull request #2 from apel/queue_purge

Outgoing queue purge